1. Field of the Invention
The present invention relates to DC to DC converters that generate a DC voltage by insulated conversion from a DC power supply voltage.
2. Description of the Related Art
FIGS. 8A and 8B show a conventional technology as disclosed in Japanese Patent No. 4094727. The main circuit of the figures composes a forward type DC to DC conversion circuit to supply a load LD with a DC power and includes a transformer Tr having a primary winding n1 and a secondary winding n2, a DC power supply DP and a MOSFET Q1 driving the primary winding n1 to conduct insulated transformation to high frequency voltage, a rectifying MOSFET Q2 and free-wheeling MOSFET Q3 to conduct synchronous rectification of the voltage of the secondary winding n2, and a filter circuit consisting of a reactor L and a capacitor Co to make the synchronously rectified voltage smooth. The tertiary winding n3 and the diode Da construct a circuit for resetting magnetic energy of the transformer Tr.
IC 4 in FIG. 8A is a controller IC (integrated circuit) to drive the MOSFETs Q2 and Q3 for synchronous rectification. The MOSFETs Q2 and Q3 each receives a driving signal from the controller IC IC4, in the time when a forward current flows in the antiparallel-connected diode, and carries an electric current in the direction from the source to drain. The voltage at this time, which is a voltage drop between source and drain of the MOSFET, is lower than the forward voltage drop of the diode, thus reducing the power loss. If the ON signal periods of the MOSFETs Q2 and Q3 are overlapping in the process of transfer from the rectifying to free-wheeling operation or in the process of transfer from the free-wheeling to rectifying operation, the secondary winding of the transformer would be short-circuited to break semiconductor elements. Accordingly, the MOSFETs are driven with a dead time to avoid the overlapping of the ON signal period. A power supply capacitor C1 is connected between a power supply terminal Vcc of the controller IC IC4 and the GND. The capacitor C1 is connected in parallel with the smoothing capacitor Co. Thus, the controller IC IC4 is supplied with DC power from the smoothing capacitor Co.
IC5 in FIG. 8B is a controller IC to drive the MOSFETs Q2 and Q3. The way of operation thereof is the same as the controller IC IC4 in FIG. 8A. A capacitor C1 is connected between a power supply terminal Vcc of the IC5 and the GND. A series-connected circuit of a resistor R3 and a diode D3 is connected between the terminal Vcc and the connection point of one terminal of the secondary winding n2 and the reactor L. Thus, the controller IC IC5 is supplied with power from the secondary winding n2 of the transformer Tr through the resistor R3 and the diode D3.
As described above, the synchronously rectifying semiconductor switch that is an antiparallel-connected circuit of a diode and a MOSFET undergoes an operation mode in which a reverse voltage is applied in the state the diode is carrying a forward current. This mode generates surge voltage due to reverse recovery of the diode. This surge voltage gives the semiconductor element a voltage stress and causes radiation noise and conduction noise. One configuration to address this problem is a snubber circuit parallel-connected to the semiconductor switch. This configuration, however, presents problems of increased number of parts and greater power loss. Thus, there is a need in the art for a DC to DC converter with relatively low power loss, that includes a relatively low number of parts.